1. Field of the Invention
The present invention relates to a process for producing an epoxidized polymer. The polymer obtained by the process of the present invention is useful as a starting material for a ultraviolet-curing resin for sealant or coating use, and as an adhesive.
2. Description of the Related Art
Epoxidation of a polymer having an olefinic carbon-carbon double bond is a convenient method to produce a polymer with an epoxy group.
As an example of such a method, epoxidation of a polymer having an olefinic carbon-carbon double bond with a peroxy acid such as performic acid is known [See xe2x80x9cPolymers for Advanced Technologies, Vol. 7, pp. 67-72 (1996)xe2x80x9d]. However, peroxy acids are comparatively expensive. Also, epoxidation with a peroxy acid has a problem that the epoxy group in the epoxidized polymer formed may be subjected to ring cleavage with acids such as formic acid which are present in the reaction mixture.
As other examples of epoxidation of a polymer having an olefinic carbon-carbon double bond, epoxidation with hydrogen peroxide, which is more economical, is described, for example, in the following documents [1] to [4].
[1] xe2x80x9cJournal of Polymer Science: Part A: Polymer Chemistry, 29, 1183-1189 (1991)xe2x80x9d [hereinafter abbreviated as xe2x80x9cdocument 1xe2x80x9d] describes a process for epoxidation of a styrene-butadiene-styrene triblock copolymer with hydrogen peroxide and trioctylmethylammonium tetrakis(diperoxotungsto)phosphate, which is isolated from the reaction mixture prepared by adding phosphoric acid and trioctylmethylammonium chloride to the mixture of tungstic acid and hydrogen peroxide.
[2] U.S. Pat. No. 5,789,512 [hereinafter abbreviated as xe2x80x9cdocument 2xe2x80x9d] describes a process for epoxidizing unsaturated polymers comprising reacting a unsaturated polymer with hydrogen peroxide in the presence of (a) tungstic acid or its metal salts, (b) phosphoric acid or its metal salts and (c) at least one phase transfer catalyst.
[3] Japanese Patent Application Laid-open No. Hei 5-247016 (JP 247016/1993) [hereinafter abbreviated as xe2x80x9cdocument 3xe2x80x9d] describes a process for producing epoxidized glycidyl esters comprising epoxidizing glycidyl esters having cyclohexene rings with hydrogen peroxide by using a system composed of (a) one or more oxidation catalysts selected from tungstic acids and molybdenic acids, (b) long-chain alkyl group-containing quaternary ammonium salts or long-chain alkyl group-containing phosphonium salts and (c) phosphate anions as a catalyst.
[4] Japanese Patent Application Laid-open No. Hei 5-247120 (JP 247120/1993) [hereinafter abbreviated as xe2x80x9cdocument 4xe2x80x9d] describes a process for producing a dicyclopentadieneepoxy (meth)acrylate polymer by the epoxidation of dicyclopentadiene (meth)acrylate polymer with hydrogen peroxide, a phosphoric acid compound, a tungstic acid compound and an onium salt, in a two phase system.
However, in the method indicated in document 1, trioctylmethylammonium tetrakis(diperoxotungsto)phosphate, that is an oxo complex of tungsten, must be isolated before the epoxidation step. This makes the total procedures complicated. In addition, the preparation step of the oxo complex and the epoxidation step both require, as a solvent, a halogenated hydrocarbon, which is considered to have a negative impact on the environment.
Document 2 shows no details of the procedures for mixing of each component used in the epoxidation reaction. Document 2 only describes, in the xe2x80x9cExamples 1 to 3xe2x80x9d, processes for producing epoxidized polybutadiene having epoxy values of 182.7 mgKOH/g (Example 1), 183.3 mgKOH/g (Example 2) and 172.8 mgKOH/g (Example 3). However, in these examples, only about 20% of the double bonds of polybutadiene were epoxidized. The present inventors tried to make the ratio of epoxidized double bonds of polybutadiene higher according to the description of document 2, but only observed the gelation of the reaction mixture and failed to obtain an epoxidized polybutadiene. Moreover, in the method described in the xe2x80x9cExamplesxe2x80x9d of document 2, considerable amount of oxygen was generated due to decomposition of hydrogen peroxide. For this reason, it is necessary to use hydrogen peroxide in a large amount. This causes a problem of safety.
Document 3 describes, in its xe2x80x9cExamplesxe2x80x9d, an epoxidation method wherein an aqueous solution of a salt of tungstic acid and phosphoric acid, the pH of which had been adjusted using sodium carbonate, was added to the solution of a substrate and a quaternary ammonium salt in an organic solvent, followed by the addition of an aqueous hydrogen peroxide under agitation. However, the alkali metal ion present in the reaction mixture promotes a decomposition of hydrogen peroxide. For this reason, it is necessary to use hydrogen peroxide in a large amount. This causes a problem of safety. The present inventors tried to apply the epoxidation method of the document 3 to a polymer having no cyclohexene ring moiety and found that, when a compound of high molecular weight was used as a substrate, gelation of the reaction mixture occurred and no epoxidized polymer was obtained.
Document 4 describes an epoxidation method wherein a solution of an onium salt in a water insoluble organic solvent was mixed with an aqueous hydrogen peroxide containing a phosphoric acid compound and a tungstic acid compound or an aqueous hydrogen peroxide containing an adduct of a phosphoric acid compound and a tungstic acid compound, followed by the addition of a dicyclopentadiene (meth)acrylate polymer. Document 4 also describes an epoxidation method wherein a solution of an onium salt and a dicyclopentadiene (meth)acrylate polymer in a water insoluble organic solvent was mixed with an aqueous hydrogen peroxide containing a phosphoric acid compound and a tungstic acid compound or an aqueous hydrogen peroxide containing an adduct of a phosphoric acid compound and a tungstic acid compound. However, continuous generation of oxygen was observed during the epoxidation, because hydrogen peroxide and a tungstic acid compound were mixed before the addition of a dicyclopentadiene (meth)acrylate polymer. This causes a problem of safety. In addition, it is necessary to use hydrogen peroxide in a large amount.
Thus, documents 1 to 4 fail to give an industrially advantageous process for producing a polymer with an epoxy group.
It is an object of the present invention to provide an industrially advantageous process for producing an epoxidized polymer with safety and efficiency.
The present inventors have made intensive studies, to achieve the above object, especially on the method described in documents 1 to 4. Many papers other than the above documents have been published on epoxidation using a tungstic acid compound, a phosphoric acid compound and hydrogen peroxide under the presence of a quaternary ammonium salt, including the active species involved the epoxidation. However, details of such epoxidation reaction is not completely understood. So, a large number of trials are still required to establish the best condition for the epoxidation reaction according to a purpose.
Finally, the present inventors have found that, when all the following conditions indicated as (a) to (d) are fulfilled, decomposition of hydrogen peroxide can be suppressed, the ratio of the epoxidized carbon-carbon double bond of the starting polymer can be increased and the epoxidized polymer can be present with good stability in the reaction mixture. In addition, the present inventors have found that, in such a case, isolation of the epoxidized polymer can be performed easily and efficiently, because the obtained reaction mixture has good tendency to separate into two phases of an aqueous layer and an organic layer.
(a) To prepare an aqueous solution containing (i) ammonium tungstate and/or tungstophosphoric acid, which is the tungstic acid compound, and (ii) phosphoric acid, which is the phosphoric acid compound.
(b) To set the proportion of above (i) ammonium tungstate and/or tungstophosphoric acid and above (ii) phosphoric acid within a specific range.
(c) To effect the epoxidation reaction by adding the aqueous solution prepared in above (a) and an aqueous hydrogen peroxide to a solution of a quaternary ammonium salt and a polymer having an olefinic carbon-carbon double bond in an organic solvent immiscible with water.
(d) To effect the epoxidation reaction under the presence of substantially no alkali metal ion.
Specifically, the present invention provides a process for producing an epoxidized polymer, wherein the epoxidation of a polymer having an olefinic carbon-carbon double bond is effected under the presence of substantially no alkali metal ion by adding an aqueous solution containing (i) ammonium tungstate and/or tungstophosphoric acid and (ii) phosphoric acid, in which the amount of (ii) phosphoric acid is not less than 0.25 moles relative to 1 gram atom of tungsten atom contained in the (i) ammonium tungstate and/or tungstophosphoric acid, and an aqueous hydrogen peroxide, separately to a solution of said polymer and a quaternary ammonium salt in an organic solvent immiscible with water.